Kaparot is a controversial practice among some orthodox Jews whereby they sacrifice a chicken prior to the Day of Atonement. It is controversial in many quarters: among the animal rights activists, among the religiously non-observant, and among biblicists.
The animal rights activists are against this practice for a variety of reasons: the most obvious reason being that the chickens' throats are being cut with a razor blade. However, they also protest this practice as being cruel because the chickens are reportedly kept in small boxes standing in the sun without food or water sometimes for up to a few days. Some also suggest that the way the chickens are secured by their wings being held back, can only cause pain and distress for the chickens.
The religiously non-observant see this practice as ghoulish and cruel, suggesting that placing sins on someone else is unfair or silly. Some simply protest it as nothing more than superstitious cruelty.
The biblicist finds this practice controversial because it sort of resembles the Day of Atonement ritual in that it captures the element of substitutionary atonement, but misses most of the details: The biblical practice of which this is a derivative is described in Leviticus 16 and includes a priest, sacred clothing, incense, a holy place, a bull, a ram and two goats; none of which are either available for or used in the kaparot ceremony.
WARNING: The video is VERY graphic!